Therapeutic monitoring of various agents present in serum and other body fluids is a rapidly growing segment of the clinical laboratory testing field. With therapeutically effective agents, the purpose of the monitoring is to indicate adjustments in dosage so as to maintain a patient at a therapeutically effective drug level. Below this level, the agent may be toxic. Such monitoring is, as will be recognized, important not only for following changes in levels of useful therapeutic agents, but also for ascertaining levels of a wide variety of physiologically active substances which may be present in body fluids including, for example, vitamins, enzymes and hormones.
Methods available for the fast and accurate quantitative or qualitative determination of biologically active substances, at low concentrations are limited in number. The physicians diagnosis of the patient or confirmation of the diagnosis frequently involves the detection and/or quantitation of one or more substances in body fluids. The ability to detect rapidly in body fluids the presence and amounts of such materials is often critical to the patient's life.
Several methods have, in the past, been used for the assay of body fluids, notably radioimmunoassay, thin layer chromatography, enzyme immunoassay and fluorescence immunoassay systems. The use of a radioimmunoassay technique suffers from several disadvantages among which are the hazards associated with or inherent in radioactive materials, associated handling problems, instability, the need for expensive equipment for the assays and the difficulties associated with the manipulation and separation of the free and bound forms of the raidoactive materials. Thin layer chromatography is often quite slow in the development of the chromatogram, is sensitive to the presence of a variety of interfering factors and suffers from the lack of reporducibility or reliability. The use of enzyme or fluorescence immunoassay requires dealing with, and knowledge of very complicated chemistry involved in their preparation, storage and usage of the required reagents. Also, the method is sensitive to variations in environmental conditions, so that the methods have not proven entirely satisfactory to the users. Nephelometry has also been employed but the incubation times required have limited the practicality of the procedure, especially for automation. Additionally, the procedure requires the use of special equipment which is not routinely avaliable in commercial laboratories. Additionally, unless the laser nephelometer is employed interference noise or background effects are so bothersome that the technique cannot be applied directly to serum. Additionally, they cannot be used at low wavelengths where one would obtain best sensitivity because of the unavoidable increase in output power which renders them dangerous. (Deverill and Reiner, Journal of Immunological Methods 38 (1980) 191-204, at page 195).